Web3 and Program Analysis

Dr. Saurabh Joshi

Principal Researcher

Supra Research

What is a blockchain?

Execution trace of a deterministic state machine

What is a blockchain?

s_1
s_2
s_3
s_4
s_4
s_5
t_1
t_3
t_5
t_2
t_4

What is a blockchain?

Programs are state transformers

x=x+1
x = 0
x = 1
x=x+1
x = 1
x = 2
x=x+1
x = 255
x = ?

Need for determinism

Without deterministic operational semantics, given the same sequence of transactions, output state may be different when executed on a different hardware, or on the same hardware at a different time

What is a blockchain?

Execution trace of a deterministic state machine agreed upon by a network of distributed, possibly heterogenous and decentralized computers

Determinism

  • Floating point arithmetic
  • Signed integer arithmetic
  • Concurrency
  • Interaction with external environment
  • ..and many more

Termination

  • All transactions are executed with a pre-determined upperbound on gas (resources) consumed
gas : Instr \rightarrow \mathbb{N}

Gas Optimization

  • Reduce the amount of gas used while retaining the same input/output behaviour
minimize \sum gas(Instr_i)
  • Gas for storage and gas for computation makes gas optimization a bit tricky

Gas Estimation

  • Provide the number of gas units that would be consumed by the transaction
  • Undecidability
  • Input context is not known until execution
  • Provide an estimate for the number of gas units that would be consumed by the transaction
  • Accurate estimation needed both for users and for the network

Compilers and Virtual Machines

  • From high level programming languages to blockchain VMs (e.g., Solidity -> EVM, Move -> Move VM, Rust/C++ -> Solana VM)
  • Enabling smart contracts written in one language to work on another VM (e.g., Fractal, A transpiler that makes Solidity work on Move VM)
  • Enabling smart contracts written in one language to work on near native assembly (e.g., Rust/C++ -> Wasm)

Safety and security of smart contracts

Example Vulnerability

function multiClaim(uint256[] memory _tickets) external {   
	uint256 totalReward = 0;     
    for (uint i = 0; i < _tickets.length; i++) {  
    	require (msg.sender == lotteryNFT.ownerOf(_tickets[i]),    “not from owner”); 
        require (!lotteryNFT.getClaimStatus(_tickets[i]), “claimed”);  
        uint256 reward = getRewardView(_tickets[i]);  
        
        if(reward>0) {
        	totalReward = reward.add(totalReward);   
            }
     }
     
     lotteryNFT.multiClaimReward(_tickets); 
     
     if(totalReward>0) {      
     	cake.safeTransfer(address(msg.sender), totalReward); 
     }    
     
     emit MultiClaim(msg.sender, totalReward);
     
     }

“If we want to be serious about quality, it is time to get tired of finding bugs and start preventing their happening in the first place.”— Alan Page

Web3 and Program Analysis

By Saurabh Joshi

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Web3 and Program Analysis

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